It is known that the performance of a PSA method can be assessed through a number of factors, which in the aforementioned example are as follows:
the yield, which is the ratio of the volume of gas (for example oxygen) produced to the volume of said gas contained in the gas mixture which is treated (volumes measured under standard temperature and pressure conditions); PA1 the productivity, which is the quantity of gas produced per unit time and unit volume of adsorbent (unit: m.sup.3 (stp)/h.m.sup.3); PA1 the specific energy, which is the energy consumed per unit volume of product gas, measured under standard temperature and pressure conditions (unit: kWh/m.sup.3 (stp)); and PA1 the investment, which is the cost of a standard plant carrying out the method (unit: FF). PA1 ES represents the specific energy PA1 p.sub.e represents the price of the energy PA1 cc represents a capital charge which covers not only the amortization, but also the maintenance, taxes, etc. PA1 I is the investment, and PA1 PA is the annual production. PA1 the ratio S/V, where S denotes the external heat exchange surface area for the assembly consisting of the mass of adsorbent and the two dead volumes, and where V is the volume of the same assembly, is selected, at a value of less than 6 m.sup.-1 ; and PA1 the production dead volume is selected at a value of between substantially 10% and substantially 60% of the volume of the mass of adsorbent. PA1 the ratio S/V, where S denotes the external heat exchange surface area for the assembly consisting of the mass of adsorbent and the two dead volumes, and where V is the volume of the same assembly, is less than 6 m.sup.-1 ; and PA1 the production dead volume has a value of between substantially 10% and substantially 60% of the volume of the mass of adsorbent.
When the parameters of a PSA plant are altered, the four factors mentioned above are generally affected in different ways. It is therefore particularly difficult to predict what the final cost of the product gas (in particular oxygen) will be, and especially since a number of relatively poorly understood physical phenomena, such as the adsorption/desorption kinetics, are affected.
The cost C of the product gas can be defined by the following formula: EQU C=((ES'p.sub.e)+(cc'I))/PA, where
C thus represents the unit cost of the product gas.